Symbols are removed from object file when optimizations are enabled

[Ecco]

Consider this very simple sample code, referred to as test.rs hereinafter:

fn main() {
}

#[no_mangle]
pub static FOO: i32 = 1;

When compiling it without optimizations, we get the following result:

$ rustc test.rs -C linker=/bin/false
$ nm test.0.o
0000000000000040 s _FOO
                 U __ZN2rt10lang_start20hacfbaccc148e05fdU4xE
0000000000000000 t __ZN4main20h82df1d8932225dfbeaaE
0000000000000010 T _main

If we enable optimizations (like cargo build does) we get this:

$ rustc test.rs -C opt-level=3 -C linker=/bin/false
$ nm test.0.o
                 U __ZN2rt10lang_start20hacfbaccc148e05fdU4xE
0000000000000000 t __ZN4main20h82df1d8932225dfbeaaE
0000000000000010 T _main

My understanding is that it is not the compiler's job to trim this (apparently) unused symbol from the object file, but the linker's. Indeed, one could imagine that the FOO symbol might be expected by another object file that would be linked into the final executable.

[japaric]

rust-lang/rfcs#1459 proposes a #[used]/#[preserve] that would force the compiler to preserve the symbol even when it's unused.

[alexcrichton]

The relevant logic for this I believe is here and may just be a bit too eager. There's a few other locations that we check for "extern fn or #[no_mangle] static", and we may just need to add it here as well.

It would probably be good to sort this out more thoroughly though as it seems like this keeps coming up!

[JinShil]

It would probably be good to sort this out more thoroughly though as it seems like this keeps coming up!

Yep, I just ran into this too. The only workaround I could find was to add a #[linkage = "external"] attribute.
See also rust-lang/rfcs#1002

[Mark-Simulacrum]

Without optimizations:

$ nm test.0.o
0000000000000000 R FOO
0000000000000000 T main
                 U _ZN3std2rt10lang_start17he6ada39161c356c2E
0000000000000000 t _ZN4test4main17hf4d700377696cbb0E

With optimizations:

$ nm test.0.o
0000000000000000 R FOO
0000000000000000 T main
                 U _ZN3std2rt10lang_start17he6ada39161c356c2E
0000000000000000 t _ZN4test4main17hf4d700377696cbb0E